[AMRadio] New Topic - moniitoring modulation help please


Geoff ars.w5omr at gmail.com
Thu Nov 14 16:53:25 EST 2013


On 11/14/2013 06:20 AM, Bry Carling wrote:
> Brett's remarks were very positive and helpful... thank you!
>
> I have a question for the seasoned AM ops here.
>
> Some of us (like me) need a little help.  I have read a lot of articles on how one goes about
> proper moniitoring of our AM signals, but I have never understood how to properly couple an
> RF sample into my oscilloscope and determine the %mod. etc.

I've got a generic Elenco 200MHz dual-trace scope that I got at a 
hamfest for like $40 or so...

I just take a piece of coax with a BNC on it, and a piece of solid 
copper-clad wire and attach it to both sides of the coax.  I put that 
near something that has some RF leakage - inside (but not too close to) 
the final cage, near the internals of an SWR bridge, close to the 
open-wire output.. -somewhere- to sample just a bit of RF, using the 
voltage settings at Max to get close, but not actually couple to a circuit.

As for how much audio to run... you'll have a difficult time with the 
DX-60, because of it being controlled-carrier, but as long as the 
negative peaks don't flatten out, rather 'just reach' the base (or 0)  
line on the scope, you should be ok.

I set my scope up so that the carrier consumes/occupies 2 full divisions 
on the graticule.  Half of the carrier above the 0/base line, half 
below.  Right smack dab in the middle.
   Audio peaks then reach above and below.  Those are the positive 
peaks.  The negative peaks are the ones that show up as less than the 
carrier, and go down to the 0 line.

The ratio of 2:1 = 100%.  So, if your carrier is occupying 2 divisions, 
and you modulate the carrier to 100% with a 1kHZ sine-wave (to the point 
where the negative part of the sine-wave just comes down to the 
base-line, and the positive peak reaches one division above and below 
the two divisions the carrier occupies, equals 100%.  Carrier = 2 
divisions, peak = 4 divisions. 4:2 reduces to 2:1 = 100%.

> Those of you who have actually DONE this and are confident with your method and
> equipment please enlighten the rest of us with something simple enough and reliable enough
> to motivate us to either build or buy the necessary equipment to check our modulation.

Don/K4KYV wrote one time that he used an 866 in the audio line 
somewhere, inside a box that was painted black on the inside so the 
flash could easily be seen.

> I think the real question some of us have is "How clean is my signal?"
>
> I could be running 200% modulation and stays within a 6 kHz bandwidth  if the signal is clean
> andI would be happy with that. I am thinking of my DX60B (or FT857D) + TenTec 422 setup
> that will soon be pumping some tall ship signals into the sky.

The discussion of asymmetrical audio comes into play, here. Modulating a 
carrier with a sine-wave to 100% is one thing.  Having typical 
asymmetrical voice patterns, and the audio equipment to produce it, 
-could- allow you to have positive peaks that are 4:1, and -not- be 
over-modulated, because the negative cycles do not cross the base line 
of the carrier.  Using a scope in the above described method would allow 
you to monitor that, easily.

-Geoff/W5OMR



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